Underwater deposits of manganese-bearing ores have been investigated for many years, but only recently have economic, strategic and environmental concerns encouraged the development of these materials as a viable alternative to conventional terrestrial mining techniques. The underwater deposits, variously referred to as marine nodules, sea nodules, ocean-floor nodules, manganese nodules, or maritime manganese ores, are distributed throughout the open oceans of the world and have also been discovered on the floor of inland lakes such as Lake Michigan.
Although the distribution of marine nodules is broad, it is also irregular, with areas of substantial concentration occurring both at great depths and on the shallower continental shelf. The characteristics of the marine nodules vary from one deposit to another, showing considerable differences in size, shape, internal structure, and composition. In general, the marine nodules are primarily agglomerates of manganese and iron oxides, containing lesser proportions of copper, nickel and cobalt compounds, and compounds of alkali and alkaline earth metals, along with traces of some two dozen metals such as molybdenum, lead, barium, vanadium, chromium, titanium, etc. The metal content of marine nodules is generally dependent upon the region from which the nodules are obtained, and economically important components vary in content within broad ranges. For example, manganese content may range from about 5 to 50 percent by weight, iron from about 5 to 40 percent by weight, copper from about 0.03 to 2 percent by weight, nickel from about 0.01 to 2 percent by weight, and cobalt from about 0.01 to 3 percent by weight.
This wide variation in metal values has complicated the refining of marine nodules and has led to the development of numerous processes for extracting the desirable materials. Straightforward physical methods for separating the metal components have not been successful due to the extremely fine-grained nature of the primary manganese oxide and iron oxide constituents. Pyrometallurgical techniques have inherent difficulties in achieving clean separations, probably due to the formation of complex metal alloys which are difficult to separate. Consequently, most effort has been directed to hydrometallurgical processes.
A hydrometallurgical process which provides a simple, direct method for selectively separating iron and manganese from marine nodules and from each other, without high processing costs, while at the same time efficiently recovering other desirable metal components, such as copper, nickel and cobalt from marine nodules, is set forth in my U.S. Pat. No. 4,123,499 entitled "Recovering Metal Values from Marine Manganese Nodules" which is hereby incorporated by reference. In accordance with the hydrometallurgical process described in my U.S. Pat. No. 4,123,499, and illustrated in FIG. 5, manganese-bearing ores such as marine nodules can be refined and various metal values efficiently recovered by first leaching the ore in an aqueous solution of nitrous and nitric acids to selectively solubilize the manganese, copper, nickel and cobalt as their respective nitrates, then separating the leach slurry from the insoluble iron oxide and gangue. The solution is then treated with manganous hydroxide in an amount at least equal to the stoichiometric value of metals other than manganese present in the solution. This treatment precipitates substantially all of the copper, nickel, and cobalt as a mixed metal hydroxide. If desired, the filtrate may optionally be further treated by the addition of hydrogen sulfide, which precipitates the remaining traces of copper, nickel, and cobalt as sulfides and assures virtually complete separation of these metals from the nitrate solution.
After filtration to remove the mixed metal hydroxide and sulfide precipitates, the solution containing manganese nitrate and alkali and alkaline earth nitrates is subjected to a thermal decomposition process to form high purity manganese oxide. It is important that this thermal decomposition process be carried out so that the manganese nitrate decomposition is complete, and so that the manganese oxide product is of high purity and is easily separated from the alkali and alkaline earth nitrate present in the mother liquor.
The present invention is directed to an improved process for the thermal decomposition of manganese nitrate to high purity manganese oxide, which is particularly advantageous in the process for recovering metal values from marine manganese nodules or terrestrial ores containing alkali and/or alkaline earth compounds as described in my issued U.S. Pat. No. 4,123,499.